In late March of 1945, with World War II still raging, President Franklin Delano Roosevelt boarded the Ferdinand Magellan, a special bullet-proof railroad car, to carry him from Washington to his retreat in Warm Springs, Georgia. Roosevelt, recently back from the summit at Yalta, was haggard and in desperate need of a rest. Allied forces were nearing Berlin, and in a month, victory would be declared in Europe. But Roosevelt wouldn’t live to see it. Two weeks later, on April 12, the 63-year-old president died suddenly of a massive cerebral hemorrhage.

It was Eleanor Roosevelt, always dignified and gracious, who broke the shocking news to Vice President Harry Truman. Then she made her way swiftly to Warm Springs, arriving just hours after Lucy Rutherfurd, FDR’s mistress, sped away. (That Rutherfurd could secretly be there in the first place points up the difference between that paparazzi-free era and our own.) Washington officials sprang into action planning a state funeral, while FDR’s staffers in Georgia struggled to find a suitable casket in the environs of a sleepy spa town.

The next morning, the train bearing Roosevelt’s body slowly set off toward his beloved home in Hyde Park, where he was to be buried. Tens of thousands of mourners lined the tracks, waiting to pay their final respects to the man who had shepherded the country through the Great Depression and three years of war. (“Everywhere grief and reverence,” recorded one passenger looking out at the somber faces.)

That three-day, thousand-mile journey is the subject of Robert Klara’s new book, FDR’s Funeral Train: A Betrayed Widow, a Soviet Spy, and a Presidency in the Balance — a tale that unfolds with all the drama that the subtitle suggests. Klara sought out declassified Secret Service documents and unearthed old newspaper accounts and passengers’ journals to piece together a gripping, highly detailed account that begins with the fateful day in Georgia, paints a moving picture of a grieving citizenry, and brings vividly to life the goings-on aboard the funeral train itself.

The first leg of the journey ended in Washington, where half a million “silent and solemn” people turned out to see Roosevelt’s casket, drawn by seven white horses in a mile-long funeral procession to the White House. By the time the train rolled out of Washington 12 hours later, it was 18 cars long and carried not only those who had been aboard earlier — FDR’s retinue, his widow and family, and his dog, Fala — but the new president, Truman, and his family; the presidential cabinet; all nine justices of the Supreme Court; and almost every top-ranking government official. A second train followed bearing a B-list of dignitaries, diplomats, and Congressmen — as well as five truckloads of flowers. In all, about 140 of the country’s most powerful people were concentrated in a few Pullman dining cars and staterooms. The Secret Service stationed soldiers along the length of the track, but security must have been woefully inadequate, considering it was wartime. As Truman’s daughter, Margaret, later wrote: “If sabotage or an accident had wrecked that train, the nation would have been crippled.”

Klara has recreated the journey in such fascinating, cinematic detail that the luxurious Pullman cars themselves almost seem like characters (he’s clearly a railway buff). We can picture a sadly unprepared Harry Truman burdened by his new responsibilities and “the first bit of information that had come to me about the atomic bomb,” as he later wrote. We see him laboring over his emergency State of the Union address, knowing full well how vital it was to reassure a stricken nation and the Allies who now depended on his leadership. Here’s poor Eleanor, devastated at having learned that Lucy Rutherfurd was with her husband when he died, and concerned about money and her own future. Elliott Roosevelt, a bomber pilot and the only one of the four sons to get home in time for the burial, was facing deployment to the Pacific, where his brothers were serving. (Eleanor had cabled them: “Father slept away. He would expect you to carry on and finish your jobs.”) Political tensions lived on, with “Roosevelt people” still in a state of shock, and “Truman people” working on affairs of state as the train rumbled through the night. The fate of Eastern Europe was hanging in the balance. Most shockingly, an undercover spy for the KGB was amid this powerful elite.

On the third day of its journey, a chilly Sunday morning, the funeral train rolled through the Hudson Valley, “winding alongside the Hudson,” as Elliott Roosevelt wrote, “on the curving route that Father had ridden so many times before.” Sorrowful locals clustered on the banks of the river, and kept vigil at stations along the way. Franciscan monks, who had walked from their monastery nearby, were among those huddled at the Garrison station, standing silently, their heads bowed. In Poughkeepsie, the shivering crowds had assembled before dawn to see their neighbor come home for the last time.

Years before, Roosevelt had chosen the spot in his Hyde Park garden where he wanted to be buried. As the coffin was lowered into the ground, West Point cadets fired a three-volley salute. After each one sounded, Fala barked. “An unconscious salute of his own to his master,” Margaret Suckley wrote in her diary. When the burial was over, President Truman, the nine Supreme Court justices, and the nation’s highest-ranking officials boarded the train in Hyde Park to take them back to their important work in Washington.

FDR’s Funeral Train: A Betrayed Widow, a Soviet Spy, and a Presidency in the Balance by Robert Klara (Palgrave MacMillan, $27)